Arsenic in its many forms is a poison and has long been recognized as such from early literature to the present. Because of its biocidal activity, arsenic has many commercial uses and has been widely used for many years for commercial purposes, notwithstanding the recognition of its hazardous properties. Many uses of arsenic cause it to enter the environment, particularly by the contamination of soil, causing widespread environmental pollution.
Environmental pollution has occurred in the vicinity of certain facilities for wood preservation with water solutions of preservatives containing metal salts through spillage of such solutions from apparatus and wood, with the result that the underlying soil has been contaminated. Use of the treated wood has also introduced arsenic into the environment. Some of these preservatives for wood are based on compounds of copper, chromium, and pentavalent arsenic. Arsenic also finds its way into soil through the manufacture and use of insecticides, defoliants and other arsenicals. It is commonly found as an unwanted contaminant during the process of refining certain metals from ores. Arsenic, in its many forms, has been the subject of much investigation and previous attempts to remove it from the environment are the subject matter of numerous patents. Attempts have been made to treat the arsenic to bind it with the soil through fixation or encapsulation, for example. Such processes are described in U.S. Pat. Nos. 4,921,538; 4,853,208; 4,723,992; 4,329,179; and 4,142,912 for example. Attempts have also been made to leach arsenic from solids as are described in the following patents: U.S. Pat. Nos. 4,891,061; 4,888,207; 4,783,263; 4,457,776; 4,420,331; (Canadian) 1,107,514; 4,244,927; 4,244,734; 4,240,826; 4,220,627; 4,218,425; 4,149,880; 4,102,976; and 2,805,936. Other miscellaneous methods are described for the attempted removal of arsenic from various substrates as described in the following U.S. Patents, for example: U.S. Pat. Nos. 3,939,249; 3,911,078; and 4,778,591.
Notwithstanding the rapt attention the presence of arsenic in soil has attracted, up until this invention a satisfactory method for cleaning arsenic-contaminated soil and returning it to its environment free of such arsenic contamination has not been available, except in limited cases.
In recent years the environmental concerns have developed a considerable amount of information concerning arsenic removal and tests have been developed which are used to monitor its presence by the environmental regulatory agencies. One particular test used as a standard for determining satisfactory removal of arsenic contamination is the Toxicity Characteristic Leaching Procedure, or TCLP analysis, as set forth in Federal Register, Vol. 51, No. 9, at 1750 (Jan. 14, 1986). The analytical methods followed to determine such TCLP values are well known and are continuously being developed. For instance, analytical methods normally used are in accordance to the Standard Methods for the Analysis of Water and Waste Water, 16th Ed., American Public Health Association (1985). Arsenic also can be determined according to Procedures for Handling and Chemical Analysis of Sediment and Water Samples, USEPA/Corps of Engineers Technical Committee (May 1981). The environmental regulatory agencies establish maximum criteria for the presence of arsenic and for the TCLP value as determined above.
It is an advantage of this process that the arsenic is removed from the soil and not merely left in the soil at unacceptable levels even though in a theoretically inert state by fixation or encapsulation as previously occurred. Even though primarily designed for arsenic, the method of this invention can be used to extract other undesirable inorganic or organic contaminants of soils, including copper and chromium, thus the soil can be returned to the environment safety and the other contaminants, now concentrated, can be satisfactorily disposed of. Other leaching processes involve the handling of potentially dangerous materials such as strong acids or bases which is avoided in the process of this invention.
Accordingly, it is the object of this invention to provide a process for the separation of arsenic contamination from soil and allow the return of the soil, free of unacceptable levels of arsenic contamination, to its intended use. It is a further object of this invention to provide a method for reclaiming soil contaminated by arsenic such that the arsenic contamination may be safely disposed of in an environmentally responsible manner without the contamination of other parts of the environment. It is further an object of this invention to provide the environmentally safe disposition of arsenic contamination of soil while returning the soil to its intended use, using only materials which are relatively safe to handle and environmentally innocuous. These objectives and advantages are accomplished by the following described invention.